In a ferroelectric memory device, a datum stored in a memory cell is read based on a reference voltage generated by a dummy cell in most cases, and earnest efforts have been made in order to improve the characteristics of the dummy cell. However, two disadvantages have been pointed out on the conventional dummy cells.
In the dummy cell comprising a ferroelectric capacitor, in which the direction of the polarization of the ferroelectric capacitor is inverted whenever the reference voltage is generated, the reference voltage becomes uncertain on account of dielectric fatigue of ferroelectric material of the dummy cell, when the reference voltage is generated many times.
In the dummy cell of the other kind, the ferroelectric capacitor in the dummy cell is designed so that its capacitance is different from that of the memory cell. However, according to this method, the method for designing the capacitor size of the ferroelectric capacitor in the dummy cell is too complicated.
As mentioned in the above, it is extremely desirable to develop a dummy cell, in which the polarization of the dummy cell is not inverted in case that the reference voltage is generated, and the ferroelectric capacitor of the dummy cell is the same as that of the memory cell.